+1

specifically, feel along the inside of the slot in the back of the seat tube. This is cut with a circular saw which tends to leave burrs on the inside. You can file them out with a rat tail or triangular file, but work upside down so any filings don't fall into the frame where they can get into the crank bearings.

Also the corners at the top of the slot sometimes get pushed in if the frame was dropped onto that end in production, so a bit of filing there often solves the problem.